This disclosure is related to a brake system of an aircraft.
Hydraulic systems in mid and large size aircrafts are used in primary systems such as flight control systems. In the mid and large size aircrafts, engine driven hydraulic pumps and the like are used as a hydraulic pressure source, because double or more hydraulic systems are needed to be used with the primary systems and the hydraulic systems are used at all flight times in mid and large size aircrafts. In these aircrafts, a quantity of fluid leakage in the hydraulic system is not controversial, because the hydraulic pumps are activated at all times in the aircrafts and the hydraulic pressure does not decrease because of the leak.
On the other hand, hydraulic systems in small business jet aircrafts are used with secondary systems like landing gear control, brake, steering, speed brake and the like in general. Therefore, in the small business jet aircrafts, hydraulic pressure source is only used in a short amount time during a flight. Especially, the hydraulic pressure source is not necessary at all during cruise, though cruise time is a large share of flight time. That is, hydraulic systems driven by electrical pumps are often employed to produce hydraulic pressure at the time of need only.
Typically, small business jet aircraft have also a pressure accumulator as a system redundancy device in preparation for failure of the hydraulic pressure pumps and the like. In the event of a pump and/or electrical system failure, accumulators hold hydraulic fluid volume enough to stop the aircraft on the ground. In this instance, the quantity of internal leakage in the hydraulic system is very important, because the volume size of the accumulator is decided with consideration of the internal leakage. Furthermore, a large amount of leakage can require the electrical motor driven pump to turn off and on frequently. Turning the electrical pump on and off frequently reduces the life of several components including the electrical motor, the hydraulic pump, hydraulic system parts, as well as electrical power generator system.
Recently, the number of small business jet aircrafts that are provided with power brake systems including a metering-pin valve and provided with anti-skid brake systems are increasing. Usually, a servo-valve is used in the anti-skid brake system, because anti-skid brake system should release brake pressure at once to realize the anti-skid function, and servo-valve is suitable for this performance. Typically, a combination of jet-nozzle and flapper-valve is used in the servo-valve. However, the quantity of internal leakage from the metering-pin valve and the servo-valve is not negligible due to the characteristic features of these valves.
There can be found infrequently that small business jet aircrafts having an electromotive drive shut-off valve which is upstream of the brake metering valve and the anti-skid valve. Electronic solenoids can be used to drive the shut-off valve. The quantity of internal leakage from that kind of shut-off valve is not negligible too. In addition, when electrical power is not provided to that kind of shut-off valve, the brake system goes out of control. Furthermore, hydraulic pressure spikes are created by the electromotive drive shut-off valve, because conditions of on and off of the electromotive drive shut-off valve are always switched immediately. The hydraulic pressure spikes, which are provided to the hydraulic system, brake system and landing gear system of the aircraft, cause large acoustic noise and affect lifespan of the hydraulic system and the like.